DESCRIPTION: The long-term goal of this research is to understand how centrosomes in animal cells nucleate microtubules and how this nucleating activity is regulated by cell-cycle and developmental processes. The protein complex to be used in this study is the recently identified gamma-tubulin ring complex (gamma-TuRC) which can nucleate and bind to the minus ends of tits nucleated microtubules in vitro. Based on a number of lines of evidence it is hypothesized that the gamma-TuRC is the major microtubule nucleator at the centrosomes. The gamma-TuRC consists of gamma tubulin and at least five currently unidentified proteins (p183, p151, p98, p82 and p75s). The four aims of this proposal are to: 1) clone and biochemically characterize the currently unknown proteins in the gamma-TuRC; 2) elucidate the structural organization of gamma-TuRC; 3)reconstitute functional gamma TuRC in vitro; and 4)determine the detailed functions of the gamma-TuRC.